Change node support to match other AMP Project repositories

[kristoferbaxter]

Do you think we should use a consistent node version policy as amphtml? Meaning only latest LTS release.

[jridgewell]

Since this is a wrapper around npm, I went with what npm 7 supports. The net change that allowed us to support back to node 10 was a promise-based fs.readFile wrapper, so I don't think it's a burden to handle.